This invention relates to a transmission power amplifier unit and, more particularly, to a transmission power amplifier unit, which has a transmission power amplifier, for compensating for non-linear distortion of the transmission power amplifier by feed-forward control.
Frequency resources have become tight in recent years and in wireless communications there is growing use of high-efficiency transmission using digital techniques. In instances where multilevel amplitude modulation is applied to wireless communications, a vital technique is one which can suppress non-linear distortion by linearizing the amplitude characteristic of the power amplifier on the transmitting side and reduce the leakage of power between adjacent channels. Also essential is a technique which compensates for the occurrence of distortion that arises when an attempt is made to improve power efficiency by using an amplifier that exhibits poor linearity.
FIG. 12 is a block diagram of a CDMA transmitter in a base-station control apparatus for encoding, multiplexing and transmitting transmit data of control and user channels. Spread-spectrum modulators 11 to 1n of respective channels (control/user channels) each have a serial/parallel (S/P) converter 1a, spreading circuits 1b, 1c and a spreading code generator 1d. The S/P converter 1a divides transmit data alternately one bit at a time to convert the data to two sequences DI, DQ, namely in-phase component (I-component) data and quadrature-component (Q-component) data, respectively. The spreading code generator 1d generates a specific spreading code that conforms to the base station and channel, and the spreading circuits 1b, 1c multiply the data DI, DQ by the spreading code to apply spread-spectrum modulation to the data.
A combiner 21 outputs an I-component code-multiplexed signal ΣVI by combining I-component spread-spectrum modulated signals VI output by the respective spread-spectrum modulators 11˜1n, and a combiner 22 outputs a Q-component code-multiplexed signal ΣVQ by combining Q-component spread-spectrum modulated signals VQ output by the respective spread-spectrum modulators 11˜1n. DA converters 31, 32 subject the outputs of the respective combiners to a DA conversion, and a quadrature modulator 4 applies QPSK quadrature modulation to the code-multiplexed signals ΣVI, ΣVQ of the I and Q components and outputs a modulated signal. An IF circuit 5 multiplies the quadrature-modulated signal and passes signal components of a prescribed intermediate-frequency band. A frequency converter 6 mixes the intermediate-frequency signal that is output from the IF circuit with a local-oscillator signal to effect a frequency conversion to a high-frequency signal (an IF→RF conversion). An RF circuit 7 amplifies the RF signal obtained by the frequency conversion, passes signal components of a prescribed high-frequency band and inputs the components to a transmission power amplifier 9 via a variable attenuator (ATT) 8. The transmission power amplifier 9 amplifies the power of the RF signal output by the variable attenuator 8 and radiates the amplified signal into space from an antenna 10.
The transmission power amplifier 9 includes, on a per-sector basis, one brancher 9a, two or three high-power amplifiers (HPA) 9b1, 9b2, 9b3, and a combiner 9c for combining the outputs of the high-power amplifiers, and is adapted so as to cover a cell of a requisite radius. A sector is a divided area obtained by dividing a 360° area surrounding the base station into a plurality of zones. For example, if the 360° area is divided at intervals of 120°, three sectors will exist.
When a new base station is established or base stations are increased in number, a transmission power controller 11 of the new base station gradually increases transmission power up to a stipulated value, as shown in FIG. 13, in accordance with a command from a monitoring control panel 12, whereby the cell is enlarged at a fixed rate. More specifically, when a station is established, the transmission power controller 11 gradually lowers the attenuation of the variable attenuator 8 from MAX to MIN taking a pre-set time TS, thereby enlarging the cell radius at a fixed rate. Control for regulating transmission power to enlarge the cell ratio at a fixed rate when a station is established is referred to as blossoming control, and control at the time of ordinary operation following the completion of blossoming control is referred to as breathing control. The goals of enlarging a cell at a fixed rate by blossoming control are as follows:
(1) to avoid concentration of load in call processing;
(2) to control the outputs of transmitter and receiver (base station and terminals) smoothly; and
(3) to mitigate the effects on terminals within the cell and on other base stations when a station is established. In other words, if a large amount of power is suddenly output to enlarge a cell when a station is established, calls from a large number of terminals concentrate at one time, call processing cannot keep pace and a variety of problems arise. Accordingly, the cell is enlarged at a fixed rate to increase the cell radius gradually.
In this case, it is necessary that transmittable and receivable areas be equalized. Otherwise reception will not be possible even if transmission is or transmission will not be possible even if reception is. For this reason, by using blossoming control, the receivable area is increased gradually at the rate at which the cell (transmittable area) is enlarged by gradually increasing transmission power. In order to increase the receivable area gradually, first noise is introduced to the receive port of the base station and it is so arranged that distant radio waves cannot be received, then noise is reduced at the rate at which the transmittable area is enlarged to thereby gradually enlarge the receivable area.
The input/output characteristic of the transmission power amplifier (main amp) constituting the high-power amplifiers 9b1 to 9b3 is non-linear, as indicated by the dashed line in FIG. 14A. Non-linear distortion arises as a result of this non-linear characteristic, the frequency spectrum in the vicinity of a transmission frequency f0 develops side lobes, as indicated by the dashed line in FIG. 14B, leakage into the adjacent channel occurs and this causes interference between adjacent channels. Feed-forward control is known as a technique which compensates for such non-linear distortion of a transmission power amplifier.
FIG. 15 is a diagram showing the structure of a high-power amplifier (HPA) which compensates for non-linear distortion of a transmission power amplifier by feed-forward control, and FIG. 16 shows the frequency spectra of various portions of the high-power amplifier in a case where two carrier signals SC1, SC2 (denoted collectively by SC) are frequency-multiplexed and transmitted.
A control unit (CPU) 20 exercises feed-forward control so as to compensate for non-linear distortion of the transmission power amplifier (main amp). When the high-power amplifier starts up (i.e., when power is introduced), a variable attenuator 21 gradually lowers its attenuation from MAX to MIN (i.e., gradually raises its gain) under the control of the control unit 20, thereby making it possible to cancel the distortion promptly. In addition, this prevents damage caused by input of excessive power to an auxiliary amplifier (sub-amp), described later.
A brancher 22 branches the carrier signals SC (A of FIG. 16) to two signal paths a, b, and a combiner 23 combines the carrier signals SC with a pilot signal SP of a prescribed frequency (B of FIG. 16). A variable attenuator 24 and a variable phase shifter 25 adjust attenuation and phase under control of the control unit 20 so as to equalize the gains of the signal paths a and b and invert phase.
A main amp 26 amplifies the output of the phase shifter 25. Noise signals SN1, SN2 (C of FIG. 16) appear at the amplifier output owing to non-linear distortion of the main amp. A brancher 27 branches, to signal paths c and d, a noise signal and the carrier signal, which includes the pilot signal, output from the main amp.
A combiner 28 combines the signal branched by the brancher 27 with a signal delayed by a delay line 29. Since control is performed so as to equalize the gains of the signal paths a and b and so as to invert phase, the combiner 28 outputs the difference between the signals that arrive via the paths a and b. Since the signal path b includes only the distortion-free delay line 29, the combiner 28 outputs the noise components SN1, SN2 and the pilot signal SP (D of FIG. 16), which occur on the signal path a, in the steady state of feed-forward control.
A variable attenuator 30 and a variable phase shifter 31 adjust attenuation and phase under control of the control unit 20 so as to equalize the gains of the signal paths c and d and invert phase. A sub-amp 32 amplifies the output of the variable phase shifter 31. A combiner 33 combines the signal, which has been branched by the brancher 27 and delayed by a delay line 34, with the output signal of the sub-amp. Since control is performed so as to equalize the gains of the signal paths c and d and so as to invert phase, the combiner 33 outputs the difference between the signals that arrive via the paths c and d. The signal path c includes only the distortion-free delay line 34 so that the noise signals SN1, SN2 and the carrier signals SC inclusive of the pilot signal SP (C in FIG. 16) enter the combiner 33 as is. Signal path d, on the other hand, inputs the noise components SN1, SN2 and pilot signal SP to the combiner 33. As a result, the combiner 33 outputs only the carrier signals SC1, SC2 (E of FIG. 16) in the steady state of feed-forward control.
The foregoing is for an ideal case in the steady state. When feed-forward control is unstable, the carrier signals SC are not removed and remain in the output of the combiner 28, and the pilot signal SP is not removed and remains in the output of combiner 33. A detector 35 detects the carrier-signal components contained in the output of the sub-amp 32 and inputs these components to the control unit 20. A detector 36 detects the pilot-signal component contained in the output of the combiner 33 and inputs this component to the control unit 20. The latter controls the attenuation and amount of phase shift of the variable attenuator 24 and variable phase shifter 25 in such a manner that the carrier-signal components detected by the detector 35 are minimized, and controls the attenuation and amount of phase shift of the variable attenuator 30 and variable phase shifter 31 in such a manner that the pilot-signal component detected by the detector 36 becomes zero. By thenceforth executing such feed-forward control, an amplified signal from which noise signals ascribable to non-linear distortion have been eliminated can be output from the combiner 33.
It is required that the high-power amplifier (HPA) having the above-described feed-forward control function be started up by turning on a power supply when the amplifier board is replaced during operation and at the time of maintenance. At start-up, an input signal (carrier signal) the level of which is near the rated level enters. Further, in feed-forward control, considerable time is required for the carrier signal contained in the output of the combiner 28 to decrease. As a consequence, there are instances where the signal having the level close to the rated level enters the sub-amp 32 and destroys the sub-amp when power is introduced. Further, in feed-forward control, a comparatively long period of time is required to settle in the steady state if a large signal is suddenly applied.
With the conventional high-power amplifier (HPA), therefore, the attenuation of the variable attenuator 21 is set to MAX at introduction of power in order to prevent destruction of the sub-amp and exercise distortion cancellation control quickly. Following the introduction of power, the attenuation of the variable attenuator 21 is reduced gradually from MAX to MIN (=0), under control of the control unit 20, from the moment the input-signal level surpasses the set level (=VFCC) near the rated level.
The high-power amplifier starts up upon introduction of power when a station is established. The above-mentioned attenuation control by the control unit 20 therefore is carried out in parallel with blossoming control at the time of station establishment, and output power SOUT of the high-power amplifier (HPA) loses its linearity at time td, which is the time at which the input power SIN surpasses the set level VFCC, as opposed to the input power SIN (see FIG. 17) that varies linearly owing to blossoming control. If the transmission power SOUT of the base station ceases varying linearly with respect to time, then it is no longer possible to exercise the original control that attempts to enlarge the cell (the transmit area) gradually in proportion to time. Ideally, it is required that control be performed in such a manner that the transmission power of the base station will be as indicated by the dashed line SIDL.
If original power control can no longer be exercised, as mentioned above, the above-mentioned goals of blossoming control when a station is established can no longer be attained. A particular problem is that since the base station that establishes the link is changed over owing to the reception power of the mobile terminal, the base station that establishes the link will be changed over frequently unless transmission power control of the base station is performed smoothly when a station is set up. A further problem is that a difference develops between the transmittable area and receivable area so that, depending upon the region, transmission is possible but reception is not, or vice versa.